In order to exhibit static stiffness, hydrostatic bearings must regulate flow into the bearing pockets with a restrictor, feedback system, or the like. This enables the bearing to counteract externally applied loads by varying the fluid pressure in individual bearing pockets. Many hydrostatic bearings in machine tool applications use fixed resistance restrictors, such as orifices or capillaries, whose resistances are nominally equal to the flow resistance out of the bearing pocket. However, in order to achieve accuracy, the restrictors' flow resistance must all be equal or of a specific ratio. Also, because, capillary resistance, for example, varies with the fourth power of the diameter, tuning all the restrictors can be time consuming. Further, because one restrictor is required for each bearing pocket, including as many bearing pockets as possible in order to enhance averaging and improve accuracy greatly increases cost. Thus, rolling element bearings are often used whenever possible in machine tools.
Hydrostatic bearings' advantages and disadvantages were recognized early, and in the 1940's self-compensating bearing systems were developed using an opposed gap as a means to regulate flow to bearing pockets located on opposite sides of the bearing. In the 1960's, bearings were introduced that employed an atypical aerostatic bearing design that achieved compensation by including grooves of a precise depth on the surface of a shaft, which acted as flow restrictors. This form of regulating the flow on the surface also eliminated the need for separate restrictors. However, in such configurations, since the grooves act as restrictors, they must be machined or etched to a very precise depth and width that is matched to the radial clearance. One example of an aerostatic bearing design is the BlockHead™ aerostatic spindle developed by Professional Instruments Corp.
In contrast to aerostatic bearings, however, hydrostatic bearings offer substantially greater load capacity than aerostatic bearings. Conventional self-compensation methods for hydrostatic bearings were thus refined and incorporated into many different types of precision grinding machines developed primarily for machines used for grinding bearing rings. Self-compensation bearings were also developed that were used mainly for precision grinding and diamond turning machines. Other refinements of self-compensation were also developed, but required either cross-drilled holes or external plumbing to route the fluid from the compensating structures on one side of the bearing to the pockets on the other side. Alternatively, elastically deforming elements were also used to tune the compensation, but these designs add complexity and cost.
Conventional self-compensated bearings, which are less prone to clogging and have fewer parts, are desirable because their primary advantage is that their stiffness is not adversely affected by bearing gaps that are smaller or larger than intended; however, their stiffness is still finite and generally lower than ball or roller bearings. As a result, servostatic bearings were developed, where the fluid flow to the pockets was actively regulated by measurement of bearing gaps and the use of servo valves to achieve “infinite” stiffness. On the other hand, the rest of the machine structure is not infinitely stiff, and a valve on every pocket can become very expensive very quickly. Thus, this attempt to improve self-compensated bearings is also not without its drawbacks.
Moreover, the previous self-compensation designs required cross-drilling or the use of external fluid lines to connect the compensator to the opposed pad. Other designs evolved this general principle to create, for example, a thrust bearing where the compensation for the thrust lands came from features on the shaft radius. This was a forerunner of the present design; however, these designs still required the groove depths to be carefully tuned to the radial clearance. Ultimately, the first true surface self-compensating bearing was created where the compensating features are located opposite the pockets, so compensation is gap independent and the compensating features are then connected to the pockets via channels on the surface of the bearing. A high speed flow theory for this design concept was developed, and showed that it was robust enough that it could even be cast, including all the pockets and compensation features. Furthermore, surface self-compensation designs evolved to create a modular profile rail hydrostatic bearing. These designs, however, still do not lend themselves to low profile rotary tables, and hence angular surface self-compensated rotary bearings were initially developed. In this configuration, the assembly of elements used was evolved from the modular profile rail hydrostatic bearing, including the vertical orientation of the restrictor element; however, a simpler more accurate design still is needed in order to make the system mass-producible.